The effects of osmotic pressure gradients on the movement of fluid between blood, brain and cerebrospinal fluid (CSF) will be studied in cats. These gradients will be imposed on both sides of the blood-brain barrier either by the intravenous infusion of, or the cerebral ventricular perfusion of anisotonic sucrose solutions. The resultant changes in the flow of solvent and solute (electrolytes and protein) into the cerebral ventricles will be measured by ventriculocisternal perfusion. The concept of the sink-action of the CSF flow as a mechanism which limits the accumulation of excess brain water will be tested in cats with experimentally induced cerebral edema caused by freeze injury, water intoxication, and ischemia. A continuous monitoring method to follow cerebral blood flow changes will be adapted for use in the cat. In this way, the time necessary for induced blood flow changes to occur can be measured. Choroid plexus blood flow will also be measured to determine whether the 3-4 fold increase in volume flow of fluid into the ventricles under the imposition of osmotic gradients is accompanied by blood flow changes. Osmotic pressure gradients will be induced by either ventricular perfusion of hypertonic sucrose solutions, or intravenous infusion of hypotonic sucrose solutions. In addition, the reactivity of choroid plexus and cerebral cortical vessels to changes in arterial blood CO2 tension will be compared. Autoregulation of choroidal blood flow in response to changes in the perfusion pressure will also be compared to that of the cerebral cortex.